1. Field of the Invention
The present invention relates to improving electroless deposition on a non-conductive substrate by increasing the catalytic activity of the catalyst on the substrate prior to electroless plating. More particularly, the present invention relates to an improvement in a method of electroless plating wherein a non-conductive substrate is treated with a catalyst and the catalyst treated substrate is then contacted with an accelerator bath which increases the catalytic activity of the catalyzed substrate.
The method of the present invention is applicable in functional applications where metal deposited on a non-conductive substrate renders the substrate thermally conductive, electrically conductive, stronger, or more rigid, or a combination of these properties. The method of the present invention may also be used in decorative applications where the substrate is plated with a metal deposit to provide a pleasing finish.
2. Description of the Prior Art
Metals are deposited on non-conductive substrates by electroless plating techniques by first rendering the surface of the non-conductive substrate catalytic toward electroless metal deposition. One type of substrate for which electroless plating techniques are used is printed circuit boards having a copper-clad epoxy glass structure with drilled through-holes. A conductive copper layer is deposited on the through-holes to electrically connect adjacent copper layers. In prior art methods, substrates are cleaned, etched or made wettable, and then treated with a solution containing catalyst. The commercially predominant electroless systems typically deposit copper or nickel wherein the metal plating baths commonly contain reducing agents, usually formaldehyde in alkaline solution for copper plating, and more recently, hypophosphite reduced baths for copper plating, and hypophosphite reduction for nickel metal. When catalyzed non-conductive substrates are immersed or brought in contact with the electroless metal baths, a metal film will result on the substrate.
Widely accepted catalyst solutions are the tin-palladium colloids such as described in U.S. Pat. No. 3,011,920 and U.S. Pat. No. 3,532,518. They are composed of acidic solutions of tin-palladium which are made colloidal upon aging and/or heating, or by other known suitable methods. In these tin-palladium activator sols, the palladium is the catalytically active material, and the tin acts as a protective colloid, stabilizing the bath. When the non-conductors are immersed in these tin-palladium colloid activator baths, the active catalyst absorbs or adheres to the non-conductive substrate.
However, the presence of the protective tin can cause problems in the electroless metal deposition step, such as lengthy metal deposition times, blistering of deposited metal to substrate and contamination of the bath with tin. Therefore, an acceleration step is added between the activation step and electroless metal deposition. The prior art accelerator bath comprises a solvent for the protective metal, being substantially a non-solvent for the catalytic metal. The result of immersion of the substrate in the accelerator bath is exposure of the catalytic surface for electroless deposition. The accelerator step is followed by water rinsing to avoid or reduce contamination of the plating bath with accelerator solution.
The "accelerator" solution used to "accelerate" the catalyzed non-conductor as described in U.S. Pat. No. 3,011,920 is an acidic or alkaline solution which is thought to act as "a solvent to remove the protective colloid and/or the deflocculating agent from the colloidal particles of catalytic metal on the substrate surface". This patent further discloses that oxidation of the treated surface should be avoided. As described in U.S. Pat. No. 3,011,920 the "accelerator" step may sometimes be omitted when using an electroless bath solution that will itself be effective to remove the stabilizing material (such as an alkaline electroless copper bath using formaldehyde reducing agent.)
U.S. Pat. No. 4,448,811 discloses the use of acidic solutions preferably having a pH less than about 1 for use as accelerating solutions. It is stated that oxidizing agents such as chromic acid in conventional acidic accelerators make such accelerators sufficiently more aggressive to assure the removal of activating species which have been absorbed on stop-off painted areas and/or plating racks. It is believed that such highly acidic solutions are too aggressive for use on copper-clad printed circuit boards inasmuch as some of the copper may be dissolved. Similarly, U.S. Pat. No. 3,533,828 discloses use of acidic accelerator solutions containing perchloric acid or paladium chloride. U.S. Pat. No. 3,562,038 also discloses treatment with acidic stripper solution such as a hydrochloric acid and sodium perborate water solution.
In certain hypophosphite reduced nickel baths, and in some more recently developed hypophosphite reduced copper baths as described in U.S. Pat. Nos. 4,209,331 and 4,279,948, insufficient coverage of electroless metal on catalyzed substrates may occur if the accelerator step is omitted. Using the standard prior art acid or alkaline accelerators with these hypophosphite baths produced coverage but problems with slow rates of metal deposition were encountered. In general it can be said that when using the commercially proven, well stabilized electroless solutions, one finds the proper "acceleration" of the substrate to be a necessary step to insure complete coverage of metal on the catalyzed surface. However, problems can be encountered with the acceleration step itself. So-called "over-acceleration" which is thought to be excessive removal of catalytic metal from the substrate, can cause skip plating or "voiding" of the electroless metal film. Since inherent surface conditions of the various non-conductor substrates cause varying amounts of catalyst adsorption over the surface of a given part, one area of the part can be over accelerated (that is, too much catalyst is removed), thus causing skip plating, whereas another area of the same part which has a more adequate catalyst adsorption due to more favorable surface topography could be properly accelerated and acceptably plated. Thus for the variety of parts plated commercially, one will inevitably obtain varying degrees of catalyst adsorption, thus narrowing and complicating the accelerator solution parameters that must be maintained to insure that, over a given part, areas low in catalyst density not get over accelerated, while avoiding the problem that more effectively catalyzed areas of the part be "under accelerated." With conventional acceleration baths, rinses following the acceleration step may become contaminated, or the electroless plating bath may become contaminated by stannous tin and operate outside desired optimum parameters. In production situations, such instances are common rather than exceptional.
It is the object of this present invention to provide improved accelerator compositions for use in a method of electroless plating that will produce a superior catalytic surface on a non-conductive substrate which is to be electrolessly plated. Another object of this invention is to provide more complete and rapid coverage on non-conductive substrates in the electroless plating bath. Still another object of the invention is to eliminate or reduce contamination of rinses and the plating bath with stannous tin (Sn.sup.+2). A further object is to reduce or eliminate acidic disolution of copper layers of a copper-clad printed circuit board during the acceleration step.